(1) Field of the Invention
This invention generally relates to guidance systems and more specifically to a method and apparatus for guiding an object, such as a submarine torpedo, from a control site, such as a launching submarine, to a remote target or contact.
(2) Description of the Prior Art
Early methods for directing an object toward a contact from a launching site included calculating a fixed trajectory in anticipation of contact maneuvers and then sending the object along that fixed trajectory path. Subsequently there were developed more sophisticated systems for downloading control programs into the steerable object, prior to launch. However, all those methods were characterized by the fact that the launching site lost any control of the steerable object immediately upon launch.
More recent guidance systems for such steerable objects, in both airborne and underwater applications, establish a communications link with the launch site so that systems at the launch site can continuously update the guidance information at the steerable object. In airborne operations this communications path generally includes a radio link between the control site and a missile. Typically control and launch sites are in close proximity so "control site" and "launch site" are used interchangeably in the following discussion.
U.S. Pat. No. 5,310,134 to Hsu et al. discloses a tethered airborne vehicle positioning system comprising a tethered vehicle body having a control system and a propulsion system therein. An optical fiber data link extends from the tethered vehicle control system to a control site. The tethered vehicle system further includes a GPS positioning system and a transmitter that transmits an amplified positioning signal that an optical fiber data link conveys to a receiver at the control site.
It is also known to use an analogous approach for underwater applications. For example, U.S. Pat. No. 3,643,616 to Jones discloses a torpedo guidance system in which a torpedo receives guidance signals for positioning the torpedo to have a substantially constant lead angle with respect to a target by adjusting torpedo speed. A control cable maintains communications until the torpedo is within range of a target or other tactical situations exist that enable the human operator at the firing vessel control to sever the control cable.
The availability of such a control cable as a link between a control site and torpedo has permitted the development of a number of sophisticated systems onboard a submarine that can produce sophisticated control trajectories. For example, U.S. Pat. No. 5,319,556 to Bessacini discloses an adaptive trajectory system apparatus and method. This system uses a plurality of measured or estimated position and motion parameters associated with a contact and contact parameters that include information on quality and alertment status. The contact parameters are compared with an information matrix that defines a plurality of trajectory strategies and implementing data parameters. A trajectory strategy is defined as a candidate trajectory strategy when the required data parameters are included in the measured or estimated contact parameters. An expert system periodically selects a unique possible trajectory strategy based upon a predetermined set of rules that utilize the received contact parameters, quality thereof, contact alertment status and vehicle state information received from the underwater vehicle. The vehicle control commands are generated with a controller using the selected candidate trajectory strategy in the vehicle state information.
This patent discloses three basic trajectories, namely bearing rider, pursuit and target intercept trajectories. A beam rider trajectory is a trajectory established when a reference point on the torpedo remains on the bearing between the control site and the contact. In a pursuit trajectory the heading of the steerable object or torpedo is controlled so as to always point at the contact. In a target intercept trajectory the course of the steerable object is set to intercept the contact or target at some time in the future. One of these trajectories typically will provide optimal results for a given set of tactical conditions.
However, the success of all these systems depends upon the integrity of the communications path including the control cable. By integrity of the communications link it is meant that the communications path faithfully reproduces guidance commands at the steerable object and faithfully reproduces at the control site any signals sent from the steerable object. Although the integrity of this path may fail because of a failure in the electronics, it is more likely that the control link in the path, either the fiber optic or radio link in the airborne applications or the cable in a submarine application, will fail due to interference or severance.
Several solutions have been proposed to overcome this problem. U.S. Pat. No. 5,133,520 to Daly discloses a missile guidance system. A siting and tracking apparatus provides output signals that are transmitted to the missile to control it along a demanded flight path. In the event that the control of the missile by the tracker is lost during flight, adjustment of the computer is immediately stopped and control signals from the model in its existing state of adjustment are employed to direct the continued flight of the missile.
Another solution particularly directed to the severance of submarine cables is disclosed in U.S. Pat. No. 5,377,164 to Almstrom et al. This patent discloses a method for operating with a wire guided movable vehicle in the form of a reconnaissance and weapon system for underwater use that comprises a vehicle, which may be a torpedo, and a support. A control wire connects the vehicle and the support. This wire has a density lower than that of water so that it tends to float. Providing the cable with buoyancy causes the cable to float above the sea bed and therefore makes the cable less prone to snaring and breakage by snagging sea bed objects.
These solutions, however, do not facilitate the employment of some of the sophisticated trajectory approaches. The use of a last guidance system as disclosed in the Daly patent is only effective if a pursuit trajectory is being used whereby the steerable object path is always pointed at the contact. The remaining trajectories could, at the time of a cable severance, have oriented the torpedo along a path that would, if followed, miss the contact. The use of a buoyant control cable might eliminate some problems. However, a buoyant control cable would not eliminate all the events that can cause control cable severance. Consequently, even though the alternate trajectories are available and may be more optimal than the pursuit trajectory for a given tactical situation, that pursuit trajectory continues to be the solution of choice because it does provide the best option if communications with the torpedo or steerable object are lost.